1. Field of the Invention
The present invention relates to a display apparatus, and particularly, to a display apparatus such as a plasma display panel (hereinafter referred to as PDP) that divides a frame period of an image signal into a plurality of subframes and selectively activates the subframes to display gray-scale images.
2. Description of Related Art
The PDP drives each pixel in a binary mode or an ON/OFF mode. To display gray-scale images, the PDP divides a frame period (16.7 ms) of an image signal into subframes having different light emitting periods, respectively. The subframes are selectively driven according to a gray-scale level to display, so that a human eye may observe a gray-scale image due to a visual integration effect. The display apparatus employing the in-frame time division displaying method is disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei-7-271325.
FIG. 1 is a block diagram showing a display apparatus according to a related art. An image signal to be displayed is supplied to an image processor 1. The image processor 1 conducts image processes including error diffusion, dithering, and inverse gamma correction. The image signal processed by the image processor 1 is transferred to a subframe coordinator 2, which converts the image signal into subframes to drive corresponding red (R), green (G), and blue (B) pixels. At this time, the subframe coordinator 2 refers to a coding table 4 and a weighting table 3 stored in an external storage device. The weighting table 3 is related to the coding table 4 and indicates the number of pulses generated in each subframe to determine the brightness of the subframe.
The signal processed by the subframe coordinator 2 is transferred to a subframe processor 5. The subframe processor temporarily stores the signal, reads a subframe at the display timing thereof, sends a control signal to a drive pulse generator 6, and provides an address electrode driver 7 with pixel data. The drive pulse generator 6 supplies drive pulses to an X-electrode driver 8 and a Y-electrode driver 9, to start sustain discharge and activate pixels selected by the address electrode driver 7. As a result, the selected pixels are activated on a plasma display panel (PDP) 10. These operations are conducted subframe by subframe.
FIG. 2 shows an example of a subframe structure used to display a gray-scale image according to the related art. In FIG. 2, an ordinate indicates display lines Y1 to Yn and an abscissa indicates time. To realize 256 (8-bit) gray-scale levels, the example of FIG. 2 divides a frame into eight subframes SF1 to SF8 having different brightness weights. An LSB (least significant bit) to an MSB (most significant bit) of 8-bit image data are sequentially assigned to the subframes. Namely, the related art divides a frame into M subframes, selects subframes according to a gray-scale level of image data, and displays a gray-scale level of the “M”th power of 2 on the PDP 10, so that a viewer may see a gray-scale image on the PDP 10 due to a visual integration effect.
Each subframe consists of a reset period, an addressing period, and a sustain discharge period. The addressing period is a period to conduct a sequential line-by-line write operation. In FIG. 2, the sustain discharge periods are depicted with patterns and have different lengths from subframe to subframe. This is because each subframe has an individual brightness weight that determines the number of sustain pulses to be generated during the sustain discharge period of the subframe. The weights, i.e., the numbers of pulses generated during the sustain discharge periods of the subframes SF1 to SF8 are 1, 2, 4, 8, 16, 32, 64, and 128, respectively. To increase the brightness of light emission, the numbers of pulses are multiplied by N (a natural number).
The number of subframes may differ depending on display apparatuses. The PDP usually employs 10 to 12 subframes depending on the reset, addressing, and sustain discharge periods to be included in a frame period.
It is known that the display apparatus employing subframes to display gray-scale images shows false contours when displaying dynamic images. The false contours displayed on dynamic images will be explained.
FIG. 3 shows adjacent pixels displaying gray-scale levels of 127 and 128 on the PDP. In FIG. 3, a vertical direction indicates the pixels displaying the gray-scale levels of 127 and 128, and a horizontal direction indicates time. Subframes depicted with patterns are those selected to emit light. In this example, there are eight subframes SF1 to SF8 that are weighted by 1, 2, 4, 8, 16, 32, 64, and 128, respectively.
For the pixel to display the gray-scale level of 127, the subframes SF1, SF2, SF3, SF4, SF5, SF6, and SF7 are driven, so that the total weight of 127 thereof provides the gray-scale level of 127. On the other hand, for the pixel to display the gray-scale level of 128, only the subframe SF8 is driven, so that the weight of 128 thereof provides the gray-scale level of 128.
If a still image is displayed on the PDP at this time, a line of sight of a viewer is immobile. Namely, the line of sight does not move to the next pixel during integration of the weights of the subframes. In this case, the image is correctly viewed, and no false contour appears. If a dynamic image is displayed on the PDP at the time, a line of sight of the viewer moves according to the movement of the image. Namely, the line of sight moves to the next pixel before the weights of the subframes of the first pixel are integrated. Then, the viewer sees a false contour due to the visual integration effect of the eyes.
In FIG. 3, the pixels to display the gray-scale levels of 127 and 128 are adjacent to each other. If a dynamic image moves upwardly and if the eyes of the viewer move from one pixel to another at a speed within a visual integration time, a line of sight of the viewer will be a line “a” shown in FIG. 3. In this case, the viewer sees a black color because there are no subframes to emit light. If the image moves downwardly, the eyes of the viewer will move along a line “b”. In this case, the subframes SF1 to SF8 are driven to emit light and their brightness is integrated, so that the viewer sees the total weight of 256, i.e., a gray-scale level of 256. In both cases, the pixels display the gray-scale levels of 127 and 128, respectively. However, the viewer sees a false white or black stripe.
This is a phenomenon called a dynamic image false contour. The phenomenon is specific to the display apparatus employing the in-frame time division displaying method and deteriorates image quality. The phenomenon, therefore, must be eliminated.
To solve the problem, there is a related art that employs two kinds of coding to realize different gray-scale levels with subframes. This related art displays 256 gray-scale levels by averaging the two kinds of coding. This technique is disclosed in, for example, Japanese Unexamined Patent Publication No. 2003-66892. The related art finds first and second gray-scale levels whose average is equal to a given gray-scale level, forms a light emission pattern A of subframes according to the first gray-scale level and a light emission pattern B of subframes according to the second gray-scale level, and alternates the light emission patterns A and B frame by frame.
This related art, however, involves some gray-scale levels each selecting the same subframes in both the light emission patterns A and B to unavoidably cause false contours.